import java.util.*;

/*
 * public class TreeNode {
 *   int val = 0;
 *   TreeNode left = null;
 *   TreeNode right = null;
 *   public TreeNode(int val) {
 *     this.val = val;
 *   }
 * }
 */

public class Solution11 {
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     * 
     * @param root TreeNode类 
     * @return int整型一维数组
     */
    public int[] postorderTraversal (TreeNode root) {
        // write code here
        if(root == null) {
            return new int[0];
        }
        ArrayList<Integer> list = new ArrayList<>();
       Stack<TreeNode> s = new Stack<>();
       TreeNode pre = null;
        while(root != null || !s.isEmpty()) {
            while(root != null) {
                s.push(root);
                root = root.left;
            }
            TreeNode cur = s.pop();
            if(cur.right == null || cur.right == pre) {
                list.add(cur.val);
                pre = cur;
            } else {
                s.push(cur);
                root = cur.right;
            }

        }
        int[] res = new int[list.size()];
        for(int i = 0;i<list.size();i++) {
            res[i] = list.get(i);
        }
        return res;
    }
    
}